The present invention relates to a printing method for application in an inkjet printer containing a substantially closed ink chamber provided with a nozzle, said ink chamber being operationally connected to an electro-mechanical converter, the method comprising: printing an image for the purposes of which the converter is actuated according to a predetermined printing strategy in order to eject ink drops from the nozzle onto a carrier for the formation of the image thereon, detecting the presence of an obstruction, for example, a gas bubble, inside the ink chamber during printing, and subsequently interrupting the printing process. The present invention also relates to an inkjet printer which has been modified so that the present method can be automatically applied.
A method of this kind is known from U.S. Pat. No. 4,695,852 (Scardovi, 1987). The printer in question comprises an ink chamber which is connected to a piezo-electrical converter. By actuating this converter, pressure waves are generated inside the ink chamber, said pressure waves in turn being able to cause ink drops to be ejected from the nozzle. By actuating the converter image-wise, an image, made up of individual ink drops, may thus be formed on a carrier. To this end, the printer comprises a calculation unit which determines an adequate printing strategy prior to printing the image. This printing strategy comprises information on the times at which and the actuation pulse with which the ink chamber is to be actuated for the correct ink drop to take on the required pixel on the carrier. In the known method, the inkjet printer comprises only one ink chamber, said ink chamber being part of a printhead which is a fixed arrangement in the printer. The carrier is led along the printhead in a number of scanning movements, so that ultimately, the entire carrier may be provided with ink drops. In other types of inkjet printers, the printhead is a movable arrangement in the printer, so that it may make this scanning movement in an initial (main) scanning direction. As the printheads usually do not extend along the entire length of the carrier, the carrier is usually moved along the printhead in a number of (sub) scanning directions. The combination of both scanning movements means that the entire carrier may be provided with ink drops, despite the limited dimensions of the printhead. As is generally known from the prior art, it will be necessary to take these scanning movements into account when determining the printing strategy, in particular when determining the exact times at which the ink drops are to be ejected from the ink chamber, as this determines the position the printhead takes on relative to the carrier at each moment during the printing process. Prior to the image being printed, a strategy is thus determined with which the image may, in principle, i.e., if no unforeseen problems occur, be printed. According to one embodiment, a total image to be printed, e.g., the image of a photograph in A3 format, is divided into a number of partial images, where the printing strategy is determined per partial image, prior to it being printed. An example of such partial image is a strip of the total image, said strip having a width that is equal to the width of an image that may be printed in one scan action of the printhead. In the known printer, a detection circuit has been provided to determine whether an obstruction, i.e., anything that hinders the inkdrop formation process, in particular a gas bubble, is present in the chamber while an image is being printed (i.e. immediately after the piezo-electrical converter has been actuated in order to provide the first pixel on the carrier with an ink drop). If such an obstruction is detected, the printing process will be interrupted, as such obstructions usually have an adverse effect on the drop formation process, for example, ink drops that are formed which are too small or which do not have the correct injection speed. This means that ink drops of an unintended size end up on the carrier or on an unintended location of the carrier, which may result in print artifacts. Furthermore, it is known that where the ink chamber is permanently loaded, an obstruction will usually produce ink chamber failure (i.e. a state in which the ink chamber is no longer able to eject an ink drop when the converter is actuated). Thus it is known that gas bubbles may grow quickly due to the pressure waves being generated. From a certain size upwards, it will no longer be possible to eject ink drops from the ink chamber since their presence interferes too much with the acoustics in the chamber such that, for example, all acoustic energy is absorbed or reflected by the gas bubble. Failure of the ink chamber will virtually always produce highly undesirable print artifacts. In order to prevent this as far as possible, the obstruction is removed from the ink chamber after the printing process has been interrupted, for example by purging the ink chamber with fresh ink, after which the printing process may be resumed.
However, the known method does have a number of major disadvantages. Interrupting the printing process is relatively time-consuming, which is considered a nuisance by any user of the printer. Furthermore, a relatively large amount of ink is required to ensure that the obstruction is indeed removed during the purging operation. Another important disadvantage is that joining errors often occur between the part of the image printed before the obstruction was detected and the part printed after the obstruction has been removed. Thus, to enable the ink chamber to be purged with fresh ink, it is moved to a purge station, so that the purging ink cannot soil the carrier. After the purging operation, the printhead will need to be repositioned in the exact same place where it was when the printing process was interrupted. Here, it is virtually impossible to prevent positioning errors in the region of 20 to 200 μm, often producing visible joining errors.